Historical Rules · 27 April 2018
Rule 209.2
Official source text from the 27 April 2018 Rules release
An activated ability with a loyalty symbol in its cost is a loyalty ability. Loyalty abilities follow special rules: A player may activate a loyalty ability of a permanent they control any time they have priority and the stack is empty during a main phase of their turn, but only if none of that permanent’s loyalty abilities have been activated that turn. See rule 606, “Loyalty Abilities.”